Many trunked radio communication systems provide at least one control channel and two or more user channels. Each channel comprises a pair of frequencies, one inbound and one outbound, where inbound messages are transmitted to a system controller and outbound messages are transmitted by the system controller. The system controller regulates the use of the user channels by receiving channel assignment requests from various subscriber units on the inbound control frequency, and transmitting a code containing the channel assignment on the outbound control frequency.
In one such system, the channel assignment code constitutes a 10 bit field in an outbound signalling word (OSW) (i.e., a word transmitted by the system controller on the outbound control frequency). The channel assignment code typically identifies only the assigned transmit frequency. Since transmit and receive frequencies are preassigned as known pairs in many countries and bands, the receiving subscriber unit can calculate both the assigned transmit and receive frequencies using a simple algorithm based on the channel assignment code. This method allows new channels to be added to a system without concurrently requiring reprogramming of all subscriber units.
This protocol works well in the current 800 and 900 MHz bands in the United States where trunked RF communications are currently offered and allowed. In these bands, the designated channel plan identifies specific frequency pairs, requires a constant offset between the transmit and receive frequencies, and also provides for a constant channel spacing within the band. The OSW bit field described above provides sufficient room to allow for the proper number of channel permutations in these bands. Further, by knowing either the transmit or receive frequency, the other can be calculated given the constant offset utilized in these bands.
Trunked RF communications, however, are now moving to other bands, including VHF and UHF. In these other bands, a constant offset between transmit and receive frequencies does not exist. Furthermore, constant channel spacing has not been provided throughout a given band. Also, within a band, the number of channels possibly available for use exceeds the number of permutations available in the current 10 bit channel field.
A need exists for allowing current trunked communication systems using 10 bit channel fields to provide proper channel assignments in these bands.